1. Field of the Invention
The present invention relates to a light emitting diode formed through growth of non-polar GaN-based semiconductor layers, and more particularly, to a non-polar light emitting diode having a barrier layer with a superlattice structure.
2. Discussion of the Background
In general, since Group-III-element nitrides, such as GaN, AlN and InGaN, have excellent thermal stability and a direct-transition-type energy band structure, they have recently come into the spotlight as materials for light emitting diodes (LEDs) in blue and ultraviolet regions. Particularly, an InGaN compound semiconductor has been noticed for its narrow band gap. LEDs using such a GaN-based compound semiconductor are used in various applications such as large-sized full-color flat panel displays, backlight sources, traffic lights, indoor illumination, high-density light sources, high-resolution output systems and optical communications.
FIG. 1 is a sectional view illustrating a conventional LED.
Referring to FIG. 1, the LED includes an N-type semiconductor layer 17, a P-type semiconductor layer 21, and an active region 19 interposed between the N-type and P-type semiconductor layers 17 and 21. The N-type and P-type semiconductor layers 17 and 21 include Group-III-element nitride semiconductor layers, i.e., (Al, In, Ga)N-based compound semiconductor layers. Meanwhile, the active region 19 has a single quantum well structure having a single well layer, or a multiple quantum well structure having a plurality of well layers, as shown in this figure. The active region 19 with a multiple quantum well structure includes alternately laminating InGaN well layers 19a and GaN barrier layers 19b. The well layer 19a includes a semiconductor layer with a smaller band gap than the N-type and P-type semiconductor layers 17 and 19 and the barrier layer 19b, thereby providing quantum wells in which electrons and holes are recombined with each other.
Such a Group-III-element nitride semiconductor layer is grown on a different-type substrate 11 with a hexagonal system structure, such as sapphire or SiC, using a method such as organic chemical vapor deposition (MOCVD). However, if a Group-III-element nitride semiconductor layer is grown on the different-type substrate 11, a crack or warpage occurs in the semiconductor layer and dislocation is produced due to the difference of lattice coefficients and thermal expansion coefficients between the semiconductor layer and the substrate.
In order to prevent these problems, a buffer layer is formed on the substrate 11. The buffer layer generally includes a low-temperature buffer layer 13 and a high-temperature buffer layer 15. The low-temperature buffer layer 13 is generally formed of AlxGa1-xN(0≦x≦1) at a temperature of 400 to 800° C. using a method such as MOCVD. The high-temperature buffer layer 15 is then formed on the low-temperature buffer layer 13. The high-temperature buffer layer 15 is formed of a GaN layer at a temperature of 900 to 1200° C. Accordingly, crystal defects in the N-type GaN layer 17, the active region 19, and the P-type GaN layer 21 can be reduced.
However, although the low-temperature and high-temperature buffer layers 13 and 15 are employed, crystal defect density in the active region 19 is still high. Particularly, to enhance a bonding efficiency of electrons and holes, the active region 19 includes a semiconductor layer with a smaller band gap than the N-type and P-type GaN layers 17 and 19. In addition, the well layer 19a includes a semiconductor layer with a smaller band gap than the barrier layer 19b. The semiconductor layer with a small band gap generally contains a large amount of In and thus has a large lattice coefficient. Therefore, lattice mismatch occurs between the well layer 19a and the barrier layer 19b and between the well layer 19a and the N-type semiconductor layer 17. Such lattice mismatch between the layers causes pin holes, surface roughness, and degradation of crystal structures.
Meanwhile, GaN and its compounds are the most stable in a hexagonal system crystal structure expressed by axes of equivalent bases, which rotate at an angle of 120 degrees with respect to each other and are all perpendicular to the unique c-axis as shown in FIG. 2. Referring to FIG. 2, as a result of positions of gallium and nitrogen atoms in the crystal structure, each plane contains only one kind of atom, i.e., Ga or N while advancing along the c-axis plane by plane. To maintain charge neutrality, GaN crystals form the boundary between one c-plane containing only nitrogen atoms and one c-plane containing only gallium atoms. As a result, the GaN crystals are polarized along the c-axis, and the voluntary polarization of the GaN crystals depends on the crystal structure and composition as bulk properties.
Since it is relatively easy to grow c-plane {0001} containing Ga atoms, almost all conventional GaN-based LEDs are grown in parallel with a polar c-axis. In addition, interface stress between different kinds of layers may additionally cause piezoelectric polarization. The total polarization is the sum of voluntary polarization and piezoelectric polarization.
The conventional GaN-based LED includes GaN-based semiconductor layers grown along a c-axis direction. However, due to strong piezoelectric polarization and voluntary polarization, c-plane quantum well structures of the LED are influenced by a quantum-confined stark effect (QCSE) in an active region. Further, electrons and holes are spatially separated by strong internal electric fields along the c-direction, so that recombination efficiency of electrons and holes is reduced.
Accordingly, there is an interest in enhancing recombination efficiency of electrons and holes in an active region and in solving problems associated with lattice mismatch between a well layer with a multiple quantum well structure and an N-type semiconductor layer.